Heretofore, it was necessary that video signals and audio signals which were digitally signalized with a predetermined format be converted to a different format. In this case, if sampling frequencies (L, M) of the digital signals were different from each other, the sampling frequencies (L, M) of the digital signals must be converted from a sampling frequency L to a sampling frequency M or, in contrast, be converted from a sampling frequency M to a sampling frequency L.
For example, an apparatus for converting sampling frequencies is a sampling rate converter. An example of a sampling rate converter is disclosed in U.S. Pat. No. 5,159,339. Functions and uses are explained in detail in this Patent.
Further, the sampling rate converter generally consists of a digital filter, an example of which is disclosed in Japanese Patent Application Laid-Open No. 42,909/89. Thus, the sampling rate converter constituted by a digital filter is known in the prior art.
Theoretically, the filter coefficient which is obtained when converting the sampling frequency from the frequency M to the frequency L, i.e., when converting by L/M times (hereinafter referred to as M:L conversion), can be the same in converting the sampling frequency from the frequency L to frequency M, i.e., when converting by M/L times (hereinafter referred to as L:M conversion).
However, in such a sampling rate converter, when the sampling rate is changed, the outputs (DC gain), in the case where direct current components (DC) are inputted in the sampling rate converter, fluctuate on every output. This is because of the quantization error when the filter coefficient is quantized.
Therefore, it is necessary to amend the value of filter coefficients after quantization in order to keep these changes, based on the DC gain, constant. In the conventional sampling rate converter, for example, when the filter coefficients are operated such that the DC gain at the M:L conversion is constant in all phases, the DC gain does not remain constant at the L:M conversion.
Accordingly, the filter coefficient on the M:L conversion differs from the filter coefficient on the L:M conversion, and in order to realize the sampling rate converter which can perform both M:L and L:M conversions, it becomes necessary to have two independent sets of filter coefficients for the respective conversions. Further, in the case where only one set of filter coefficients is available based on hardware restrictions, the DC gain is not constant in either conversion periods.